Try here:
https://www.mobileread.com/forums/sho...d.php?t=200703
I have tried it with a Class 10 16GB MicroSDHC and it works, but I changed it back because a) I don't have enough books to fill it up the original card and b) I found no noticeable increase in responsiveness.
Also, make sure that you don't wipe or write to the original MicroSD. That way you always have a fallback in case something goes wrong.
Happy reading!